Recently, a display device represented by a liquid crystal display device often is equipped with a photo sensor for automatically adjusting brightness of a display screen in accordance with the intensity of light surrounding the display device. A display device having a plurality of photo sensors arranged in a matrix also has been known. In this display device, the plural photo sensors function as one area sensor, thereby capturing an image at the observer side.
The incorporation of the photo sensor in the display device can be achieved by mounting a photo sensor as a discrete component on a display panel thereof. Alternatively, a photo sensor can be formed monolithically on an active matrix substrate by utilizing a process for forming an active element (TFT) or a peripheral circuit.
In the field of a display device for a mobile terminal device in particular, the photo sensor is required to be formed monolithically on the active matrix substrate, from the viewpoint of reducing the number of components and downsizing the display device. As the photo sensor formed monolithically, a photodiode formed of a silicon layer, for instance, is known (see, for instance, JP 2006-3857 A: FIGS. 2 and 3).
Hereinafter, a conventional photodiode (photo sensor) will be explained with reference to FIG. 12. FIG. 12 is a cross-sectional view showing a configuration of a liquid crystal display panel having a photodiode. As shown in FIG. 12, a photodiode 51 is a PIN diode having a lateral structure, which is formed monolithically on an active matrix substrate 50 that forms a liquid crystal display panel.
As shown in FIG. 12, the photodiode 51 includes a silicon layer 60. The silicon layer 60 is formed on a glass substrate 52 as the base substrate of the active matrix substrate 50, by utilizing a process of forming a thin film transistor (TFT) that functions as an active element, at the same time of forming the TFT. Further on the silicon layer 60, an n-type semiconductor region (n-layer) 51a, an intrinsic semiconductor region (i-layer) 51b and a p-type semiconductor region (p-layer) 51c are formed in this order along the planar direction. The i-layer 51b serves as a photodetection region of the photodiode 51.
On an under layer of the photodiode 51, a light-shielding layer 53 for shielding light from a backlight device (not shown) is provided. The light-shielding layer 53 is covered with an insulating base coat 54. Typically the light-shielding layer 53 is formed of a metal material. The light-shielding layer 53 is in an electrically suspended state, insulated from the ambience. The photodiode 51 is covered further with interlayer insulating layers 55 and 56.
In FIG. 12, numeral 57 denotes a wiring connected to the n-layer 51a, and 58 denotes a wiring connected to the p-layer 51c. Numeral 59 denotes a flattening layer, and 61 denotes a protective layer. Numeral 62 denotes a liquid crystal layer. For a filter substrate 63, only the appearance is shown.
In the example as shown in FIG. 12, since the metal light-shielding layer 53 is arranged on the under layer of the photodiode 51, the output characteristics of the photodiode 51 fluctuate with the fluctuation in an electric potential of the light-shielding layer 53. And the electric potential of the light-shielding layer 53 fluctuates in connection with the fluctuation in an electric potential of the photodiode p-layer 51c. 
However, the light-shielding layer 53, the photodiode 51 and the other films positioned in the vicinity of the photodiode 51 contain a fixed charge captured during the formation process. The amount of the fixed charge varies among the photodiodes or the active matrix substrates, and the difference in the fixed charge results in the difference in the relationship between the electric potential of the light-shielding layer 53 and the output characteristics of the photodiode. Namely, in a case where a plurality of photodiodes 51 of the same specification are provided, the output characteristics may vary among the photodiodes even though an equal voltage is applied to the respective p-layers 51c and the electric potentials in the respective light-shielding layers are set to be equivalent.
The relationship between the electric potential of the light-shielding layer 53 and the output characteristics of the photodiode will vary due to not only the fixed charge but the variations in the diffusion concentration of impurities in the semiconductor regions of the photodiode 51. Similarly to the above-described case, the output characteristics may be varied among the photodiodes even though an equivalent voltage is applied to the respective p-layers 51c. 
As mentioned above, in the example as shown in FIG. 12, a problem that the output characteristics vary among the photodiodes arises. Specifically for instance, the output characteristics are different from one product to another even for the photodiodes of the same specification. Alternatively, the output characteristics are different from one photodiode to another even for photodiodes of the same specification mounted on the same active matrix substrate. In such a case, it will be difficult to adjust the brightness of the display screen with the photo sensor or to capture a high-resolution picture.